This invention relates to biaxially oriented polypropylene film and method for its production, in which rigidity of the biaxially oriented polypropylene is improved.
Biaxially oriented polypropylene is used as a packing material or an industrial material because of its excellent transparence, moisture barrier properties, good workability, etc. However, copared with cellophane which competes with biaxially oriented polypropylene, the latter is inferior to the former in rigidity. Accordingly, the latter is not always a perfect material.
By improving rigidity of the biaxially oriented polypropylene film, it can be produced rigidly and thinly, so that its cost may be reduced. Further, such an improved polypropylene film biaxially oriented may become a more suitable material for printing machines, packing machines, etc. Accordingly, a way to improve the rigidity of the biaxially oriented polypropylene film has been considered in this industry.
Conventional methods for improving the rigidity of the biaxially oriented polypropylene film will now be referred to briefly. Any one of them means a post process of biaxial orientation of the polypropylene film.
According to Japanese Examined Patent Publication No. 34-5887, biaxially oriented polypropylene film is again oriented in a longitudinal direction.
According to Japanese Examined Patent Publication No. 49-18628, it is again oriented in a longitudinal direction for a long distance.
According to Japanese Examined Patent Publication No. 46-9677, the polypropylene film is again oriented in a longitudinal direction by a double-stage process.
Further, according to Japanese Examined Patent Publications Nos. 43-26110, 54-61269 and 54-66978, it is oriented in a longitudinal direction and then biaxially at the same time.
However, any one of the conventional techniques needs a larger device in addition to the existing equipment, a broader space therefor and consequently incurs extra cost. Further, the biaxially oriented polypropylene film which has been obtained by each of the above conventional techniques has an improved property in the longitudinal orientation, but its property in the lateral orientation is greatly degraded. Finally, a certain amount of imbalance is caused between the longitudinal rigidity and the lateral rigidity, thereby the film's stability is lost. In order to remove the disadvantages of the conventional art, this invention has been accomplished.